103605 What makes us human

Editor's note: The following is a summary of this week's Time magazine cover story.

(Time.com) -- You don't have to be a biologist or an anthropologist to see how closely the great apes -- gorillas, chimpanzees, bonobos and orangutans -- resemble us.

Even a child can see that their bodies are pretty much the same as ours, apart from some exaggerated proportions and extra body hair. Apes have dexterous hands much like ours but unlike those of any other creature. And, most striking of all, their faces are uncannily expressive, showing a range of emotions that are eerily familiar.

It isn't just a superficial resemblance. Chimps, especially, not only look like us, they also share with us some human-like behaviors. They make and use tools and teach those skills to their offspring.

They prey on other animals and occasionally murder each other. They have complex social hierarchies and some aspects of what anthropologists consider culture. They can't form words, but they can learn to communicate via sign language and symbols and to perform complex cognitive tasks.

Scientists figured out decades ago that chimps are our nearest evolutionary cousins, roughly 98 percent to 99 percent identical to humans at the genetic level. When it comes to DNA, a human is closer to a chimp than a mouse is to a rat.Yet tiny differences, sprinkled throughout the genome, have made all the difference.

Agriculture, language, art, music, technology and philosophy -- all the achievements that make us profoundly different from chimpanzees -- are somehow encoded within minute fractions of our genetic code.

Nobody yet knows precisely where they are or how they work, but somewhere in the nuclei of our cells are handfuls of amino acids, arranged in a specific order, that endow us with the brainpower to outthink and outdo our closest relatives on the tree of life. They give us the ability to speak and write and read, to compose symphonies, paint masterpieces and delve into the molecular biology that makes us what we are.

Until recently, there was no way to unravel these crucial differences. Exactly what gives us advantages like complex brains and the ability to walk upright -- and certain disadvantages, including susceptibility to some types of malaria, AIDS and Alzheimer's, that don't seem to afflict chimps -- remained a mystery.

But that's rapidly changing. Just a year ago, geneticists announced that they had sequenced a rough draft of the chimpanzee genome, allowing the first side-by-side comparisons of human and chimpanzee DNA.

Already, that research has led to important discoveries about the development of the human brain over the past few million years. It's also led to more controversial theories about our ancestors' mating behavior, most notably the possibility that chimps and humans interbred from time to time before finally going their separate evolutionary ways.

And sometime in the next few weeks, a team led by molecular geneticist Svante Pääbo of the Max Planck Institute for Evolutionary Anthropology, in Leipzig, Germany, will announce an even more stunning achievement: the sequencing of a significant fraction of the genome of Neanderthals -- the human-like species we picture when we hear the word "caveman" -- who are far closer to us genetically than chimps are.

And while Neanderthals went extinct tens of thousands of years ago, Pääbo is convinced he's on the way to reconstructing the entire genome of that long-lost relative, using DNA extracted, against all odds, from a 38,000-year-old bone.

None of this is easy to pull off. Like any complex organic molecule, DNA degrades over time, and bones that lie in the ground for thousands of years become badly contaminated with DNA from bacteria and fungi. Anyone who handles the fossils can also leave human DNA behind.

But laid side by side, these three sets of genetic blueprints -- plus the genomes of gorillas and other apes, which are already well on the way to being completely sequenced -- will not only begin to explain precisely what makes us human but could lead to a better understanding of human diseases, including malaria, AIDS and viral hepatitis, and how to treat them.

For most of us, though, it's the grand question about what it was that made us human that make comparative genome studies so compelling. As scientists keep reminding us, evolution is a random process in which haphazard genetic changes interact with random environmental conditions to produce an organism somehow fitter than its fellows.

After 3.5 billion years of such randomness, a creature emerged that could ponder its own origins -- and revel in a Mozart adagio. Within a few short years, we may finally understand precisely when and how that happened.